Collaborative RFID reader using code divisional multiple access (CDMA) and methods for same

ABSTRACT

The present disclosure provides collaborative radiofrequency identification (RFID) readers that employ code division multiple access (CDMA) encoding to simultaneously broadcast to and read responses from tags in an overlapping reading zone with improved data synchronization and read yield rates. In some embodiments, a harmonic backscattering scheme is used to enable the system to have a much higher signal-to-noise ratio (SNR) and sensitivity, while the reader CDMA protocol can be integrated with an initial TDMA polling process or alternative tag CDMA scheme.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.62/671,405, filed on May 14, 2018, the disclosure of which isincorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with government support under DE-AR0000528awarded by the Department of Energy. The government has certain rightsin the invention.

FIELD OF THE DISCLOSURE

The present disclosure relates to RFID systems, and in particular,simultaneous RFID signaling by multiple collaborative readers using CDMAreader encoding.

BACKGROUND OF THE DISCLOSURE

Read reliability is one of the critical limits of ultra-high-frequency(“UHF”) radio frequency identification (“RFID”) systems when a largenumber of passive tags are present in a complex multi-path environment.In many practical applications, read failure rates often should be lowerthan 10⁻⁵, or equivalently, read yields should be above 99.999%, toachieve manageable logistic purposes in Internet of Things (“IoT”).Because of the directivities of reader and tag antennas and RF shadowingfrom objects and other tags, a tag at a certain position and/ororientation may not be able to harvest sufficient RF energy from areader to power up, or the tag may not effectively backscatter to thereader even if it is within the nominal reading range.

Previous RFID systems with the electronic production code (“EPC”)generation 2 (Gen 2) protocol perform a detailed tag singulation processin time division multiple access (“TDMA”), explore multiple readerantenna placement, and introduce relative movement between the readerand tags to minimize the read failure probability, but with only limitedsuccess. Alternatively, to improve the read yield or to enlarge thetotal coverage area, one can disperse more readers with significantlyoverlapped reading zones. However, this has the potential for severereader-to-reader collision (“R2RC”) that is difficult to resolve withinthe current EPC tag TDMA scheme. This is because when multiple readersare employed, further collaborative reader TDMA schemes need to beadopted by all readers to resolve R2RC, as not only the receiver (Rx) ofone reader can be interfered by the other reader transmitters (Tx), butalso the tags within the reading zones of more than one readers cannotdecode the reader commands correctly when they broadcast at the sametime.

As shown in FIG. 1, the reading zone of a first reader (Reader 1) mayonly cover Tag Group 1, missing Tag Group 2. Adding Reader 2 to coverTag Group 2 may cause interference (e.g., Reader 1 Rx can be interferedby Reader 2 Tx), and tags in both reading zones cannot properly decodethe reader commands. Even in tag-talk-first (“TTF”) and tag-talk-only(“TTO”) schemes, with the reader playing a smaller role in streamliningthe tag response sequence, R2RC remains a serious concern to the systemoperation. Additional delay caused by tag TDMA embedded inside readerTDMA will also limit the tag sampling rate for tracking moving tags inreal time.

BRIEF SUMMARY OF THE DISCLOSURE

In a first aspect, the present disclosure provides an RFID system usinga collaborative reader code division multiple access (“CDMA”) protocol.In this way, the RFID system provides synchronous tag access for allreaders within range. The tag can distinguish the downlink commands frommultiple readers simultaneously and then formulate a responseaccordingly.

The present disclosure provides a collaborative reader CDMA scheme in aharmonic RFID system, which can solve the reader-to-reader collision(“R2RC”) problem and significantly enhance the read yield rate for tagsin the overlapped reading zone. The reduction in the read failure ratein the collaborative reader CDMA scheme is much more than the failurerate product of the individual readers in the complex multi-pathscenarios by exploiting the channel characteristics and the failurecorrelation. This improvement can fundamentally benefit the reliabilityin the logistic RFID applications.

The presented reader CDMA is demonstrated in a simple experimentalprototype to verify R2RC resolution and read yield improvement. Thetwo-reader collaborative CDMA scheme achieved below 10⁻⁵ in the readfailure rate when individual readers had 0.05-0.2 failure rates in thecomplex multi-path scenarios due to rich channel exploitation andanticorrelation in the failure incidences.

DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the nature and objects of the disclosure,reference should be made to the following detailed description taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a diagram depicting how two nearby RFID readers can sufferfrom reader-to-reader collision (“R2RC”) for tags within the overlappedreading zone as well as possible reader receiver jamming.

FIG. 2 is a schematic of a harmonic RFID system. LPF: low-pass filter;HPF: high-pass filter.

FIG. 3(a) is a schematic of a harmonic RFID tag with harmonic generationby a reflective nonlinear transmission line (“NLTL”) and having an RFswitch for uplink modulation.

FIG. 3(b) is a PCB prototype of the harmonic tag of FIG. 3(a).

FIG. 4 is a diagram of an exemplary harmonic RFID system according to anembodiment of the present disclosure and having two CDMA readersconfigured by a software-defined radio (SDR) platform (showing whereReader 1 and Reader 2 can simultaneously access a harmonic tag within anoverlapped reading zone).

FIG. 5 is a diagram of an exemplary RFID reader according to anotherembodiment of the present disclosure and shown with additional readersand a harmonic tag.

FIG. 6 is a flow chart showing an exemplary collaborative reader CDMAprotocol.

FIG. 7 is a chart showing a method according to another embodiment ofthe present disclosure.

FIGS. 8(a)-8(b) are illustrations of an exemplary reader CDMA protocol.(a) The information for the readers to be sent to the tag. (b) Theorthogonal CDMA codes of each reader. (c) The baseband waveform of eachreader and a possible baseband waveform received by a tag when thereceived signal strength (RSS) from the two readers is similar.

FIG. 9 shows an exemplary experimental setup where an SDR module isconfigured as two harmonic readers operating the collaborative CDMAprotocol implemented by LabVIEW. A harmonic tag stands on a movable foamsubstrate for convenience. The ranges of the Ant1 and Ant2 are just forconvenient picture illustration. Experiments were conducted with bothreaders at about 2 meters away from the tag.

FIGS. 10(a)-10(c) show details of the experimental prototype for readerCDMA: (a) The modulated digital IF signals of the two readers. Thecurves are R1I (lower solid line): the in-phase signal of Reader 1, R1Q(lower dashed line): the quadrature signal of Reader 1, R2I (upper solidline): the in-phase signal of Reader 2, and R2Q (upper dashed line): thequadrature signal of Reader 2. (b) The baseband received at the tag. (c)The baseband received from Ant1 of Reader 1 and Ant2 of Reader 2.

FIGS. 11(a)-11(d). Experimental scenarios with various scatterers in thereading zone. (a) Both readers maintain direct line-of-sight to a tagwhen it moves and rotates randomly within the overlapped reading zone.(b-d) Large scatterers are successively added to the reading zone tocreate complex multi-path scenarios.

FIGS. 12(a)-12(d). Experimental benchmarks of the failure rates in thesingle reader scheme and the collaborative CDMA reader scheme for thechannel conditions in FIGS. 11(a)-11(d). The dotted line shows the slopeof 2 in the log-log plot, which corresponds to the case of uncorrelatedindividual reader failures and little reader collaboration, which ismostly valid for both curves in (a). Slopes higher than 2 in (b)-(d)suggest anticorrelation in individual reader failures and rich readercollaboration. Failure rates of below 10⁻⁵ were achieved usingcollaborative reader CDMA even though individual readers have failurerates between 0.05 and 0.2.

DETAILED DESCRIPTION OF THE DISCLOSURE

In a first aspect, the present disclosure provides collaborativeradiofrequency identification (RFID) readers that employ code divisionmultiple access (CDMA) encoding to simultaneously broadcast to and readresponses from tags in an overlapping reading zone with improved datasynchronization and read yield rates. In some embodiments, a harmonicbackscattering scheme is used to enable the system to have a much highersignal-to-noise ratio (SNR) and sensitivity, while the reader CDMAprotocol can be integrated with an initial TDMA polling process oralternative tag CDMA scheme.

In an embodiment, a method 100 for collaborative RFID is provided (see,e.g., FIG. 7). An RFID reader is provided 103. The provided 103 readerhas a baseband frequency and is a member of a group of participatingRFID readers. Each participating RFID reader is synchronized to thebaseband frequency. For example, each participating RFID reader may havea common baseband clock, CLK_(bb). The provided 103 reader is assignedan orthogonal code with which it can produce a CDMA-encoded signal. Eachparticipating RFID reader is assigned a code, and each of the codes areorthogonal to the other codes (mutually orthogonal). The set of allmutually-orthogonal codes for the participating RFID readers is known toeach of the participating RFID readers, including the provided 103 RFIDreader.

The provided 103 RFID reader is used to transmit 106 a CDMA-encodedsignal to all RFID tags within a reading range of the RFID reader (i.e.,broadcasting the CDMA-encoded signal). As will be further describedbelow, a tag within range of the broadcast signal will decode the signaland may send a response. The method 100 includes receiving 109 aresponse signal from an RFID tag. The received 109 response signal willinclude data encoded with a code of a participating RFID reader. TheRFID reader will decode 112 the data of the received 109 response usingthe appropriate code of the known set of mutually-orthogonal code. Forexample, in some cases, the received 109 response signal will includedata encoded with the code of the provided 103 RFID reader, which inother cases the received 109 response signal will include data encodedwith the code of another participating RFID reader. In this way, anyRFID reader of the participating system will be able to read signalsgenerated by tags in response to the broadcast of any RFID reader of theparticipating system.

In some embodiments, the method 100 include transmitting 115 a pollingsignal to RFID tags within the reading range, the polling signalcomprising the set of the orthogonal codes of all participating RFIDreaders.

In a more particular non-limiting example used to illustrate anembodiment, at least two RFID readers are provided. Each of the at leasttwo RFID readers have a synchronized baseband frequency. For example,the diagram of FIG. 4 depicts a common baseband clock, CLK_(bb),received by a first RFID reader (Reader 1) and a second RFID reader(Reader 2). Each RFID reader is assigned a unique code which isorthogonal to the codes assigned to the other RFID readers. The set ofmutually-orthogonal codes for all of the RFID readers (of the at leasttwo RFID readers) is known to each RFID reader.

Data is encoded using the orthogonal code assigned to the first RFIDreader. In this way a first CDMA-encoded signal is can be produced usingthe orthogonal code of a first RFID reader of the at least two RFIDreaders. The first CDMA-encoded signal is transmitted using the firstRFID reader. The signal may be transmitted to an RFID tag. The same ordifferent data may also be encoded using the orthogonal code assigned toa second RFID reader to produce a second CDMA-encoded signal. The secondCDMA-encoded signal may be transmitted by the second RFID reader. Thesecond CDMA-encoded signal may be transmitted by the second RFID readerat the same time or a different time as the first CDMA-encoded signal istransmitted by the first RFID reader. Additional CDMA-encoded signalsmay be produced and transmitted using additional RFID readers and thecorresponding orthogonal codes.

A response signal may be received from the RFID tag. For example, theRFID tag may, upon receiving the signal transmitted from the first RFIDreceiver, transmit a response signal (for example, the tag maybackscatter a signal). Such a response signal may be received at an RFIDreader, such as, for example, the second RFID reader. The responsesignal includes data encoded with the orthogonal code of one or moreRFID readers, such as, for example, the orthogonal code of the firstRFID reader. Because each RFID reader has available the set ofmutually-orthogonal codes, the receiving RFID reader is able to decodethe data of the response signal regardless of which orthogonal code thedata was encoded with. In the example where the response signal isreceived by the second RFID reader and encoded with the orthogonal codeof the first RFID reader, the second RFID reader may decode the data ofthe response signal. Furthermore, the response signal may include dataencoded by, for example, the orthogonal codes of the first and secondRFID reader and decode data using both codes. In this way, regardless ofwhich reader receives a response signal from an RFID tag, the datarelevant to each reader may be decoded and processed accordingly.

In another aspect, a collaborative RFID reader 10 is provided (see,e.g., FIG. 5). The RFID reader 10 includes a transmitter (Tx) 20 havinga Tx baseband frequency modulated on a Tx carrier frequency. The Txbaseband frequency is configured to be synchronized to a Tx basebandfrequency (i.e., Tx baseband frequencies, as the case may be) of one ormore additional RFID readers 90. The Tx baseband frequencies may besynchronized by, for example, provision of a common baseband clocksignal or a baseband clock retrieval module. The RFID reader 10 furtherincludes a non-transient memory 42. The non-transient memory 42 may beany type suitable for electronic storage of data. Some examples ofnon-transient memory 42 include dynamic or static random accessmemories, flash memories, electronically-erasable programmable memories,or the like. The non-transient memory 42 is configured to store a uniqueorthogonal code assigned to each RFID reader 10,90 within thecollaborative reader CDMA scheme.

The RFID reader 10 includes a processor 40 that is in electroniccommunication with the transmitter 20 and the non-transient memory 42.The processor 40 may be, for example, a field-programmable gate array(FPGA), a microprocessor, an application-specific integrated circuit(ASIC), or the like, or combinations of these and/or other components.The processor 40 is programmed to encode data according to the assignedorthogonal code. In this way, a first CDMA-encoded signal is produced.The processor 40 transmits the first CDMA-encoded signal using thetransmitter 20. For example the first CDMA-encoded signal may betransmitted to an RFID tag 95 within range (the “reading range”) of theRFID reader 10.

The RFID 10 reader may also include a receiver (Rx) 30 in electroniccommunication with the processor 40. The receiver 30 has an Rx basebandfrequency, which is synchronized to the Tx baseband frequency. The Rxcarrier frequency may be different than the Tx carrier frequency. Forexample, the Rx carrier frequency may be twice the Tx carrier frequency(second harmonic). In such embodiments, the processor 40 may be furtherprogrammed to receive a response signal from an RFID tag 95. Theresponse signal includes encoded data, which may be encoded using theorthogonal code of the RFID reader 10 or orthogonal codes of the one ormore additional participating RFID readers 90. Because the non-transientmemory 42 has stored the set of mutually-orthogonal codes for eachreader 10,90, the RFID reader 10 can decode the encoded data of theresponse signal using the assigned orthogonal code of the RFID reader 10and/or one or more codes of the set of mutually orthogonal codes.

In another aspect, a method for collaborative RFID reading is provided.At least two RFID readers are provided. For example, multiple RFIDreaders may be provided. The multiple RFID readers have a synchronizedbaseband frequency. For example, the diagram of FIG. 4 depicts a commonbaseband clock, CLK_(bb), received by a first RFID reader (Reader 1) anda second RFID reader (Reader 2). Each RFID reader is assigned a uniquecode which is orthogonal to the codes assigned to the other RFIDreaders. The set of mutually-orthogonal codes for all of the RFIDreaders (of the participating RFID readers) is known to allparticipating RFID readers. In this way, CDMA-encoded signals areproduced. The CDMA-encoded signals are transmitted to all RFID tagswithin reading range of the RFID readers.

Response signals are received from the RFID tags. For example, the RFIDtags may, upon receiving the transmitted CDMA-encoded signals, transmitresponse signals (for example, the tags may backscatter signals). Suchresponse signals may be received at one or more of the participatingRFID readers. Because each RFID reader has available the set ofmutually-orthogonal codes, the receiving RFID readers are able to decodethe data of the response signals regardless of which orthogonal code thedata was encoded with.

The following discussion provides additional embodiments, includingprototypes, and discussion of the theory of operation, all of which areintended to be non-limiting and provided for the purpose of furtherillustrating the disclosure.

Reader CDMA in a Harmonic RFID System

In a conventional RFID system employing the EPC protocol, where thedownlink and uplink signals share the same frequency band, poorisolation by the duplex circulator with direct leakage from reader Tx toRx as well as by the antenna reflection from imperfect impedance matchand nearby large objects renders low SNR at the reader Rx. Forapplications that rely on accurate amplitude and phase demodulation ofthe backscattered signal to retrieve, for example, location and vitalsigns in addition to the digital tag identification (ID), theconventional EPC scheme cannot provide feasible performance. Embodimentsof the present disclosure may thus use a harmonic RFID system, forexample, as shown in FIG. 2, which utilizes the second harmonic toisolate the downlink (reader-to-tag) and uplink (tag-to-reader) signalsand has been demonstrated for indoor localization, tag CDMA forsimultaneous access, and vital-sign monitoring. The downlink signal fromthe reader Tx at f goes through a low-pass filter (LPF) to broadcast tothe harmonic tag which harvests the RF energy, powers up, converts partof the RF signal at f to the 2^(nd) harmonic at 2f, and finallybackscatters to the reader Rx at 2f in the uplink signal. A high-passfilter (HPF) at the reader Rx provides high isolation (sufficientseparation) between the downlink self-interference at f and the receivedharmonic signal at 2f. The reader Rx SNR is hence not limited by thehigh phase noise skirt of its own Tx. SNR and sensitivity of the readerRx can thus be greatly improved.

An exemplary harmonic tag prototype is shown in FIG. 3(a). The tagharvests the downlink RF energy through antenna A (Ant A). After the tagpowers up, the tag Rx demodulates the reader command from the downlinkand the logic unit responds according to the air protocol. Antenna B(Ant B) converts part of the downlink signal to 2 f by the reflectivenonlinear transmission line (NLTL). The harmonic signal is thenbackscattered from Ant B to the reader Rx. An RF switch in front of theNLTL may be provided for the uplink on-off keying (OOK) modulation. ThePCB prototype of an exemplary harmonic tag is shown in FIG. 3(b), whichis based on the WISP platform. Because the tag is passive, its size andpackaging are not limited by the battery or the recharging circuits.Thus, the PCB prototype can be readily adapted to integrated circuits(IC) and printed antennas to reduce the overall size.

As shown in the exemplary embodiment of FIG. 4, two harmonic readers areconfigured by a software defined radio (SDR, Ettus X310, UBX 160) todemonstrate how collaborative reader CDMA can resolve R2RC and improveread yield rates. The given SDR platform provides two pairs of Tx/Rx,which are designed as two coherent but independent harmonic readers withsynchronized basebands. The FPGA in Reader 1 generates the basebandsignal transformed by the digital-to-analog converter 1 (DAC1) to theintermediate frequency (IF), which is further upconverted by the mixerto the fundamental RF band at f. The signal is then amplified by thepower amplifier (PA1) as the Tx1 signal. The low-pass filter 1 (LPF1),splitter1, and high-pass filter 1 (HPF1) forms the broadband duplexerfor Antenna 1 (Ant1) with reasonable insertion loss compromise. Thesignal flow of the duplexer is shown as the solid black arrows for thefundamental frequency signal (f) and the dotted-line arrows for the2^(nd) harmonic signal (2f). The harmonic tag responds to Reader 1 byAnt1, going through splitter1 and HPF1 to Rx1. The signal is amplifiedand down-converted by the local oscillator (LO) to the IF band, and thensampled by the analog-to-digital converter 1 (ADC1), which is thendemodulated by the FPGA to retrieve the tag OOK information. Anotherchannel of the SDR is configured as Reader 2, which operates similarlybut independently to Reader 1.

The LO synthesizers for f and 2f are driven by the same clock source asCLK1 for Reader 1 and CLK2 for Reader 2. This clock distribution makesthe harmonic reader coherent, and accurate phase of the backscatteredsignal can be retrieved for estimation of time of flight (ToF). However,for protocols such as the pulse interval encoding (PIE) used in the EPCdownlink, CLK1 and CLK2 are not required to be synchronized if therelated carrier phase information between Reader 1 and Reader 2 is notessential. However, to realize CDMA on the multiple readers, thebaseband signals of each reader should be synchronized (CLK_(bb)) withprecise bit alignment to maximize the orthogonality among chip codes.

An exemplary protocol conducted on the CDMA readers is described in FIG.6. First, through initial reader coordination, each reader will beassigned a unique chip code, and all readers within the collaborativeCDMA scheme will have the full chip code table. Then each reader willpoll as well as register its chip code to all tags within the readingrange. For each reader polling procedure, the tags can respond to thereader by an EPC TDMA protocol or the alternative tag CDMA protocol.After all readers finish the polling process, the readers send the CDMAcoded commands simultaneously to access all the tags. Each tag receivesand demodulates the CDMA coded frame, retrieving the information fromeach reader within the feasible range and responding to the readersaccordingly.

The reader CDMA modulation and demodulation scheme is illustrated inFIG. 8(a), where Reader 1 prepares to send (111001)₂ represented by thesolid line, and Reader 2 to send (100111)₂ represented by the dashedline. In this illustrative and non-limiting example, 1 bit per symbol isconsidered here. As shown in FIG. 8(b), Reader 1 has the CDMA chip code(01)₂ and Reader 2 the code (10)₂. In FIG. 8(c), the solid line is thebaseband signal for Reader 1, and the dashed line is for Reader 2. AfterCDMA chip code injection, the baseband has 2 bits per symbol. Thebaseband signals of these two readers are synchronized and fed to theirown DACs simultaneously. After the up-conversion of the mixers, the RFsignals of the two readers may be transmitted to the tags at the sametime, and added together in the communication channel to reach the tagRx in their respective reading zones. The dotted line in FIG. 8(c)represents a simulated baseband signal received by a tag that is in anoverlapped reading zone where the received signal strength (RSS) fromboth readers is assumed to be similar for illustrative convenience. Todemodulate the information from each reader, the tag accesses the chipcode table established during the polling stage, multiplies the selectedcode with the received baseband, and then operates all the bits (2 bitshere) within the same symbol with logical OR. The informationcorresponding to the selected chip code may then be demodulated. Forexample, the baseband signal received by the tag may be multiplied withthe orthogonal code (10)₂ of Reader 2 to obtain (100000101010)₂. Afterthe OR operation in each symbol, the original information of (100111)₂may be retrieved from Reader 2.

Experiments and Data Analyses

FIG. 9 shows the experimental setup for an exemplary collaborativereader CDMA for verification. In this non-limiting example, an SDRmodule is configured as two independent harmonic readers connected toAnt1 and Ant2, respectively. A harmonic tag stands on a foam and moveswithin the reading zones of both readers. The reader antennas are onaverage at about 2 meters away from the tag. The distances of Ant1 andAnt2 in FIG. 9 are non-limiting and provided for clear picturepresentation.

Reader 1 and Reader 2 utilize an embodiment of the present collaborativeCDMA protocol to access the tag simultaneously. In this experiment, thesame reader information and chip code assignment shown in FIG. 8 wereused. The experimental setup used a bit rate of 400 kbps (kilobits persecond), although other baseband bandwidth will not change ourobservation significantly. The IF frequency was set at 1 MHz, so eachbit spanned 2.5 periods of the IF cycle. The sampling rates of thereader Rx ADC and Tx DAC were both at 20 MSps (mega-symbols per second),where the waveform of each bit was represented by 50 points to providesufficient equalization and timing accuracy. As shown in FIG. 10(a), thelower solid curve and the lower dashed curve are the in-phase andquadrature (I/Q) signals of Reader 1 denoted by RH and R1Q, while theupper solid curve and the upper dashed curve are I/Q signals of Reader 2denoted by R2I and R2Q, respectively. The basebands of the two readerswere synchronized with the SDR, and the bits were aligned and convertedby their own DACs to the IF band, which were further up-converted to thedownlink RF signal at 950 MHz to be transmitted to the tagsimultaneously. The tag Rx was designed as the low-pass filter to detectthe envelope of the downlink signal. After the comparator, the logiclevel was equalized, as shown in FIG. 10(b). Based on the orthogonalcode table and the CDMA decoding method described above, the tagdemodulated the information from Reader 1 as (111001)₂ and from Reader 2as (100111)₂ correctly.

The tag then backscattered the 2^(nd) harmonic as the uplink signal tothe readers. In this experiment to demonstrate the collaborative readingproperties, the tag replayed what was received from all readers inrange, but the uplink can be alternatively operated with tag CDMAprotocol and tag-specific ID information for multi-tag access as well.The demodulated baseband signals from the tag to the two readersreceived by Ant1 (lower) and Ant2 (upper) are shown in FIG. 10(c).Because the distance from the tag to Ant1 and Ant2 was about the samehere, the amplitude envelopes were similar, and the waveform amplitudeswere normalized to the full scale of the ADC. The logic values aredenoted as the digits in FIG. 10(c), which are (110101101011)₂. Becausethe tag just replayed the signals from both readers on the 2^(nd)harmonic, each reader could receive the original information from bothreaders. For example, in FIG. 10(c), the baseband (upper) received byAnt2 can be decoded with the chip code of either reader, where the logic1's by the black color are attributed to Reader 2, and the gray 1's areto Reader 1. The same case applies to the baseband signal received byAnt1 from the tag. As the reader information can be processed by the tagand relayed to other readers, the multiple readers can be viewed as ahigh channel-efficiency multi-static reader system.

One of the most important applications in the multi-static reader systemis the collaborative reading under complex multi-path scenarios, becausethe channels are now not limited to the individual reader-tag-readerpath. The multi-static n-reader system yield in the channel condition Ccan be described as the matrix A(C):

$\begin{matrix}{{A(C)} = \begin{pmatrix}a_{11} & \ldots & a_{1n} \\\vdots & \ddots & \vdots \\a_{n1} & \ldots & a_{nn}\end{pmatrix}} & (1)\end{matrix}$The read yield for a specific tag can be defined by a_(ji) (i,j=1, 2, .. . , n) for the downlink signal from Reader i Tx and uplink signal toReader j Rx. Under a given channel condition C, if the tag informationcan be correctly demodulated, a_(ji)=1, otherwise a_(ji)=0. For thesingle reader system, the system read yield A(C) is just the incidenceof a₁₁ under various channel conditions C, which highly depends on therelative range, antenna orientation alignment, multi-path, and RFscattering object placement. In a multi-static reader system without theproposed CDMA protocol, the read yield can be calculated as the ORoperation of the diagonal elements of A(C), which means at least onereader should successfully read the tag. The static read yield in thiscondition is also equivalent to one reader with multiple TDMA antennasin the current EPC operations. However, one can see that A(C) is notfully utilized, and the proposed CDMA protocol can further improve thesystem yield by exploiting the OR operation of all elements in A(C). Ina complex multi-path ambient, the incidence of a_(ji)=0 can often beanti-correlated with those of other elements in the consideration ofantenna orientation alignment, which can make the system read failureprobability significantly lower than the product of individual readerfailure probability.

For example, four readers will have four Tx/Rx pairs. With thecollaborative reader CDMA protocol, all four reader Tx can poll the tagsimultaneously. The tag can then formulate the response accordingly withthe knowledge of all reader information, which can be received by allfour reader Rx. Hence, the channel information can be fully utilized toserve the various purposes in specific applications. An experimentalillustration is shown in FIGS. 11(a)-11(d) with two readers in amulti-static Tx/Rx indoor setup. The four multi-path scenarios arecreated with different scatterers (black rectangles) placed in theoverlapped reading zone (gray oval). In FIG. 11(a), both readersmaintain direct line-of-sight (LoS) to the tag when it moves and rotatesrandomly within the reading zone. In FIGS. 11(b-d), large scatterers aresuccessively inserted into the reading zone to increase the multi-pathcomplexity.

FIG. 12 shows the ample read yield improvement of the collaborativereader CDMA scheme over that of the individual readers operatingsimultaneously, especially in the complex multi-path scenarios. Noticethat Rx jamming between the two readers is removed in all cases due tothe use of harmonic backscattering and reader CDMA. The reader Tx poweris continuously adjusted to collect changing failure rates in varioustag positions and orientations. More than 300,000 reading incidences arecollected in each set of experiment data. In FIGS. 12(a)-12(d), thecurves delineated with ∘ markers and x markers are the failure rates ofReader 1 and Reader 2, respectively, vs. that of the collaborative readin the corresponding channel condition in FIGS. 11(a)-11(d). The dashedline in each figure has a slope of 2 denoting the condition ofuncorrelated read failure incidences of individual readers. In FIG.12(a), the failure rate of the collaborative reader CDMA scheme is closeto the product of the failure rates of the two readers, and the slope inthe log-log plot is around 2, indicating the expected condition ofuncorrelated read failures of the two readers. In FIGS. 12(b)-12(d),when large scatterers were successively added to the reading zone, theyield of the individual reader starts to degrade much faster than thatof the collaborative reader CDMA scheme. In FIG. 12(d), LoS of bothreaders are mostly blocked that the individual readers can only achievefailure rates of 0.2 (read yield of 80%) and 0.13 (read yield of 87%)under the largest Tx power, i.e., the diagonal terms of A(C) haverelatively low probability to be 1. In comparison, the collaborativereader CDMA scheme can still reach below 10⁻⁵ failure rate (readyield>99.999%), which implies the off-diagonal terms of A(C) contributesignificantly and their incidences of 0 are anti-correlated to those ofthe diagonal terms. Although the degree of improvement in read yieldsdepends on the actual channel condition, the slopes in the collaborativeCDMA scheme are in general much higher than 2, similar to the cases inFIGS. 12(b)-12(d), due to the rich channel availability and theanti-correlations of the read failure in each channel.

Although the present disclosure has been described with respect to oneor more particular embodiments, it will be understood that otherembodiments of the present disclosure may be made without departing fromthe spirit and scope of the present disclosure.

We claim:
 1. A method for collaborative RFID readers, comprising:providing an RFID reader, the RFID reader having a baseband frequencyand being a member of a group of participating RFID readers each beingsynchronized to the baseband frequency, and wherein the RFID reader isassigned an orthogonal code to produce a code division multiple access(CDMA)-encoded signal, and a set of mutually-orthogonal codes for allparticipating RFID readers is known to the RFID reader; transmitting,using the RFID reader, a CDMA-encoded signal to RFID tags within areading range of the RFID reader; receiving a response signal from anRFID tag within the reading range, wherein the response signal comprisesdata encoded with an orthogonal code of an RFID reader of the group ofparticipating RFID readers; and decoding the data of the response signalusing the known set of mutually-orthogonal codes.
 2. The method of claim1, wherein the response signal is decoded using the assigned code ofanother participating reader.
 3. The method of claim 1, furthercomprising transmitting a polling signal to RFID tags within the readingrange, the polling signal comprising the set of the orthogonal codes forall participating RFID readers.
 4. The method of claim 1, wherein theresponse signal is decoded using the assigned code of the RFID reader.5. The method of claim 1, further comprising synchronizing, by the RFIDreader, a baseband signal with a baseband signal of anotherparticipating reader.
 6. The method of claim 5, wherein the CDMA-encodedsignal is transmitted using the synchronized baseband signal.
 7. Themethod of claim 6, wherein the CDMA-encoded signal is transmittedsimultaneously with a CDMA-encoded signal of another participatingreader.
 8. A collaborative RFID reader, comprising: a transmitter havinga Tx baseband frequency, wherein the Tx baseband frequency is configuredto be synchronized to a Tx baseband frequency of participating RFIDreaders; a non-transient memory configured to store an assignedorthogonal code and a set of mutually-orthogonal codes assigned to allparticipating RFID readers; a processor in electronic communication withthe transmitter and the non-transient memory, wherein the processor isprogrammed to: encode data according to the assigned orthogonal code toproduce a CDMA-encoded signal; and transmit the CDMA-encoded signalusing the transmitter.
 9. The collaborative RFID reader of claim 8,further comprising: a receiver having an Rx baseband frequency, whereinthe Rx baseband frequency is synchronized to the Tx baseband frequency;wherein the processor is further programmed to: receive a responsesignal from an RFID tag, wherein the response signal comprises encodeddata; and decode the encoded data using the assigned orthogonal codeand/or one or more codes of the set of mutually orthogonal codes. 10.The collaborative RFID reader of claim 9, wherein an Rx carrierfrequency is separated from a Tx carrier frequency to reduce receiverinterference.
 11. The collaborative RFID reader of claim 10, wherein theRx carrier frequency is twice the Tx carrier frequency.
 12. An RFID tag,comprising: a non-transient memory configured to store a uniqueidentification code and a set of mutually-orthogonal RFID reader codes;and an antenna configured for a signal at a first frequency; a receiverin electronic communication with the non-transient memory and the firstantenna, the receiver configured to receive and decode a readerCDMA-encoded signal from the first antenna using a code of themutually-orthogonal RFID reader codes.
 13. The RFID tag of claim 12,further comprising an energy-harvesting circuit configured to harvest RFenergy from a signal received from the first antenna.
 14. The RFID tagof claim 12, further comprising: a second antenna configured for asignal at a second frequency, wherein the second frequency is separatedfrom the first frequency to reduce receiver interference at a reader;and a frequency conversion unit configured to convert a portion of areceived signal to a signal at a second frequency to backscatter thesignal using the second antenna.
 15. The RFID tag of claim 14, whereinthe second frequency is a harmonic of the first frequency.
 16. The RFIDtag of claim 14, wherein the frequency conversion unit comprises anonlinear transmission line.
 17. The RFID tag of claim 14, furthercomprising an RF switch for on-off keying of an uplink signal at thesecond frequency.
 18. A system of RFID readers, comprising: two or moreRFID readers according to claim 8, wherein each RFID reader of the twoor more RFID readers has a Tx baseband frequency synchronized to a Txbaseband frequency of each other RFID reader of the two or more RFIDreaders.
 19. The system of RFID readers of claim 18, wherein theprocessor of each RFID reader of the two or more RFID readers is furtherprogrammed to transmit a polling signal, the polling signal comprisingthe set of the orthogonal codes for all RFID readers of the two or moreRFID readers.
 20. The system of RFID readers of claim 18, wherein theprocessor of each RFID reader of the two or more RFID readers is furtherprogrammed to transmit the CDMA-encoded signal simultaneously with aCDMA-encoded signal of another RFID reader of the two or more RFIDreaders.